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> its a variable length opcode encoding with a complex decoder path

In practice, the performance impact of variable length encoding is largely kept in check using predictors. The extra complexity in terms of transistors is comparatively small in a large, high-performance design.

https://web.archive.org/web/20210709071934/https://www.anand...

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Jim Keller has a storied career in the x86 world, it isn't surprising he speaks fondly of it. Regardless:

>So fixed-length instructions seem really nice when you're building little baby computers, but if you're building a really big computer, to predict or to figure out where all the instructions are, it isn't dominating the die. So it doesn't matter that much.

Well, efficiency advantages are the domain of little baby computers. Better predictors give you deeper pipelines without stalls which give you higher clock speeds - higher wattages

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